Let's consider how it would be handled by a retailer such as Amazon or Best Buy, and how it might be handled similarly by Massdrop.
The product was not as advertised...
1) You are allowed to return it to Massdrop for a full refund.
2) If you keep the product, Sennheiser handles cord length as a warranty issue (Massdrop is out of the loop).
I see that the 650 comes with a user manual (https://en-us.sennheiser.com/global-downloads/file/4636/HD650_Manual_11_2016.pdf), and it says the package includes a 3 m cable. Does the HD6XX come with a user manual, and does it say anything about the cable? I won't receive mine until later in the week.

Is it actually an HD 650 user manual (does the front page say 650 or 6XX)? If the manual doesn't say 6XX, it wouldn't be considered to be completely valid. Does the box list the contents and describe the cable?